1. Field of the Invention
The present invention concerns a method of jointing a member made of Ti-Al alloy which mainly consisting of the intermetallic compound, TiAl, and a member made of a structural steel to produce one machine structural part. The invention also concerns the jointed machine parts produced by this method.
2. State of the Art
As the material for a machine part which moves rotationally or reciprocally at a high steed, such as turbin blades and engine valves or wheels of turbochargers, the use Ti-Al alloys, particularly those containing 32-38% of Al, with or without modifying elements. These alloys are heat resistant (durable at a temperature of 700.degree. C. or higher) and light weight (specific gravity 3.8) will become more popular.
The inventors have made research with coworkers on the materials for the above have developed and disclosed some Ti-Al alloys. These include the alloy which consists essentially of 32-38% of Al and the balance of Ti; the alloy which consists essentially of 32-38% of Al, 0.005-0.20% of B, and the balance of Ti and the alloy which essentially consists of the above quantity of Al and contains, in addition to the above quantity of B, up to 0.2% of C, up to 0.3% of 0 and/or up to 0.3% of N, provided that O+N up to 0.4% and the balance of Ti (Japanese Patent Disclosure Sho 63-125634). These alloys have improved strength and toughness when compared with the conventional Ti-Al alloys.
Further examples of improved alloys are the alloy which consists essentially of 32-38% of Al, 0.05-3.0% of Ni and/or 0.05-3.0% of Si and the balance of Ti and the alloy according to this composition further containing the above quantity of B or controlled amount of C, O and N (Japanese Patent Disclosure Sho 62-236609). These alloys are of higher ductility and easy to cast.
In order to make it possible to use the Ti-Al alloy member under the conditions of a high temperature and high speed movement such as rotation, it is occasionally necessary to joint the member to a member of structural steel which can be surface hardened. The term "structural steel" means the steels which can be used as the material for the structural member of machine parts. For example, in a hot wheel of a turbocharger, SCM-steel or SNCM-steel is used for the axis of rotation, and in an engine valve, martensitic heat resistant steel is used for the stem. All the steels of this kind are included in the structural steels.
As a method for jointing the Ti-Al alloy member and the structural steel member, shrink fitting has been tried, but, silver alloy brazing is usually used. However, due to the fact that the temperature of the exhaust gas from the engines has become higher due to the increased output of the engines, brazing is becoming less reliable because the strength of brazed parts is not sufficient.
On the other hand, Ni-based superalloy also used as has been a material for the hot wheels, and friction welding has been used for jointing such a wheel to the axis made of structural steel. The friction welding provides a higher strength to the jointed part (in comparison with the welding method using a filler metal), which is 95% or higher of the strength of the base metal, and therefore, it is a preferable welding method for manufacturing this kind of machine part.
Even if the friction welding is tried to joint the hot wheel made of a Ti-Al alloy and the axis made of a structural steel such as SCM 435, the jointing is not successful because cracks occur after the welding is done. This is due to breakage of the jointed part because of martensitic transformation occured in the material of the axis during cooling after the welding at a high temperature generated by the friction resulting in expansion of volume, and because of the formation of brittle substances such as TiC at the jointed region.